This invention relates generally to machinery for cutting vegetation, and more particularly to a cutting head for use with a string trimmer.
A string trimmer is a device for cutting vegetation by rotating a length of flexible cutting line about an axis at a high rotational rate, on the order of 10,000 rpm. At this high rotational speed, the flexible line extends outwardly from the rotational axis under the influence of centrifugal force with sufficient rigidity that the line can cut tall grass and other relatively thin-stemmed plants. String trimmers enjoy widespread use due to their greater safety as compared with trimmers having rotating blades. Although the rotating cutting line in a string trimmer can sever the stems of small plants, it cannot inflict serious injury on a human hand or foot that accidentally comes in contact with the flexible line, unlike a blade trimmer.
In a typical form, a string trimmer includes a tubular shaft with handles placed at convenient locations and extending downwardly to a gasoline or electric motor and cutting head assembly. A short length of cutting line extends outwardly from the cutting head, and the motor rotates the cutting head rapidly to extend the cutting line outwardly by centrifugal force to cut vegetation as the string trimmer is carried about with the cutting head just above ground level. In its simplest form, the cutting head provides an anchor for holding a short length of the cutting line. However, this simplest form of cutting head requires that the cutting operation be suspended to replace the cutting line in the event that it breaks. Consequently, several designs have been developed for replacing the broken line by extending an additional length of cutting line from the cutting head without turning off the string trimmer.
In one solution to the problem of supplying additional cutting line, a supply of flexible line is wound onto a spool which is attached by an adapter to the motor drive shaft. A housing attached to the adapter covers the spool and includes an opening through which the cutting line can extend. A clutch mechanism locks the housing and the spool together until the housing is tapped against the ground to feed additional line outwardly. When the cutting head is tapped against the ground, the clutch is unlocked so that the spool rotates faster than the housing and feeds an additional short length of line to replace that previously broken off. Upon releasing the contact against the ground, a further short length of line is fed outwardly from the spool and the clutch mechanism is reengaged to prevent further feeding of the cutting line.
Although this "touch and go" string trimmer does feed cutting line, its design suffers from several practical disadvantages. Most importantly, both the spool and the housing are attached to an adapter which is in turn attached to the motor drive shaft, so that the additive effect of tolerances in and clearances between the conventionally plastic parts is sufficient to cause significant unbalance and vibration at high rotational speeds. Additionally, since the spool is bolted to the adapter, replacement of the spool requires special tools, which can be misplaced between uses. Thus, such string trimmers can be uncomfortable and inconvenient to use. Accordingly, there has been a need for an improved "touch and go" string trimmer having reduced vibration and a simplified design for conveniently replacing the spool upon which the flexible line is wound. The present invention fulfills this need, and further provides related advantages.